From the standpoints of energy problems and environmental issues, it is demanded to efficiently reduce CO2 using light energy as in plants. Plants use a system called “Z-scheme” which excites light energy in two steps. Through photochemical reactions of this system, plants oxidize water (H2O) to obtain electrons and reduce carbon dioxide (CO2), thereby synthesizing cellulose and sugars.
However, known technologies for decomposing CO2 by obtaining electrons from water through artificial photochemical reactions without using any sacrificial reagent all have very low efficiency.
As photochemical reactors, for example, there are known devices which comprise an electrode for oxidation reaction that generates oxygen (O2) by oxidizing H2O and an electrode for reduction reaction that generates a carbon compound by reducing CO2. In such devices, the electrode for oxidation reaction is provided with an oxidation catalyst for oxidation of H2O on the surface of a photocatalyst and obtains an electric potential through light energy. The electrode for reduction reaction is provided with a reduction catalyst for reduction of CO2 on the surface of a photocatalyst and is connected with the electrode for oxidation reaction via an electric wire. The electrode for reduction reaction can generate a reduced product by obtaining a CO2 reduction potential from the electrode for oxidation reaction and thereby reducing CO2. In this manner, the use of a Z scheme-type artificial photosynthesis system mimicking plants has been examined for acquisition of an electric potential required for performing CO2 reduction using visible light and a photocatalyst.
Such devices, however, generally have very low solar energy conversion efficiency at about 0.04%. This is believed to be attributed to low energy efficiency of the photocatalyst excited by visible light. Such a very low solar energy conversion efficiency may also be attributed to that the electrode for reduction reaction is connected to the electrode for oxidation reaction via an electric wire and the electricity (electric current) extraction efficiency is thus reduced due to the wire resistance, resulting in a low efficiency.
Moreover, there have been also examined devices in which a silicon solar cell is used for obtaining a reaction potential and a catalyst is provided on both sides of the silicon solar cell to induce a reaction. Such devices may be able to achieve a high solar energy conversion efficiency of about 2.5%. In addition, such devices may take a structure requiring no wiring and can thus be easily increased in size.
In these devices, however, no case of successful CO2 reduction reaction has been known. In order to allow the CO2 reduction reaction to proceed, a further contrivance is necessary since it is required that positively charged ions generated on the side of the oxidation electrode and negatively charged ions generated on the side of the reduction electrode be allowed to migrate to the opposite electrodes. Particularly, for non-use of a sacrificial agent in a redox reaction where H2O is used as an electron donor, migration of protons (hydrogen ions (H+)) or hydroxide ions (OH−) is indispensable.
In addition, CO2-reducing activities in various metal electrodes have been reported. In the CO2 reduction reaction, electrons and protons react with CO2 and hydrocarbons such as carbon monoxide (CO), formic acid (HCOOH), methanol (CH3OH) and methane (CH4) are produced. The hydrocarbons produced by a reduction reaction vary depending on the number of electrons obtained by the reduction reaction. For instance, carbon monoxide and formic acid are produced by a reaction with two electrons, methanol is produced by a reaction with six electrons, and methane is produced by a reaction with eight electrons. In any of these hydrocarbon-producing reactions, the standard redox potential is substantially the same as that in a reaction of reducing hydrogen ions to produce hydrogen. However, a large overvoltage (excess energy) is actually required for a reduction reaction of a first electron, and this makes the reduction reaction unlikely to proceed. In addition, the larger the number of electrons required in a reduction solution, the more difficult to allow the reduction reaction to proceed with a high Faraday efficiency. In order to perform the CO2 reduction reaction for generating a hydrocarbon(s) of interest in a selective and highly efficient manner, a highly active electrode catalyst is required.
Moreover, there is known a method of performing CO2 reduction reaction using an electrode obtained by fixing a noble metal catalyst on a substrate via organic molecules. More specifically, when Si or the like is used as a material of the substrate, the substrate absorbs light and separates charges to generate electrons. The thus generated electrons are transferred to the noble metal catalyst made of Au or the like through the organic molecules on the substrate, and CO2 is reduced on the noble metal catalyst. Such a method achieves a relatively high reaction efficiency; however, there is a demand for a catalyst with which a higher reaction efficiency is attained.